c41a6e9b1b feat: add sign fn for sign_message (ChrisCho-H)
Pull request description:
While it's not hard to create the signature using `secp256k1` modules with `signed_msg_hash`, it's much more convenient and safe to provide one-way function to generate signed signature(even without the understanding about the semantics of bitcoin message signing).
ACKs for top commit:
apoelstra:
ACK c41a6e9b1b successfully ran local tests
tcharding:
ACK c41a6e9b1b
Tree-SHA512: 84caea275059381040c71100badb54556dd105722f79ac43d2df7eb0e5428cf8e7acc2d7f262625dd008837099a928c3c8be858f9ab11838c2eec1786e9f1844